Reaction times are still rarely reported in developmental psychology although they are an indicator of the neural maturity of children's information processing system. Competence and capacity are confounded in development, where children may be able to reason, or remember, but are unable to cope with information processing load. Furthermore, there are social implications in ignoring the speed factor. Slow and apparently delayed reactions from infants and children often try the patience of parents and teachers alike, and can be interpreted as non-compliance already before the child gets the chance to respond. Furthermore, individual differences in reaction times in young children are high, with standard deviations that may prevent significance of differences between task conditions. This inaugural article argues that reaction time research with children nevertheless can be done and raises the methodological problems involved. An example is presented that shows that challenges to capacity reveal true mental effort in a memory task when delay is controlled down to milliseconds rather than just in terms of the number of delay items.